This application relates in general to drive systems for threaded fasteners, tools for their manufacture, and drivers for applying torque to such fasteners. More specifically this application relates to fasteners that are constructed with straight walled recesses. In particular a fastener system is constructed wherein the driver and fastener engage with improved stability of axial alignment and stick fit.
Threaded fasteners commonly used in industrial applications typically are driven by power tools at high speeds and under high torque loads. Such conditions present difficult design considerations, particularly with respect to the drive systems and, more particularly, with threaded fasteners having a driver engageable recess in the fastener head or a driver engageable exterior contour to the fastener head. Ideally, such a drive system needs to be easily manufactured, both as to recess and head geometry, as well as to associated tooling for forming the fastener head and the drivers for engaging the recess or head geometry. The strength of the head of the fastener should not be adversely affected by the recess. The driver, when mated, should distribute the stress loads uniformly to avoid formation of highly localized regions of stress that might result in deformation of the drive surfaces, or driver, or both, leading to premature failure of the drive system.
The fastener system should resist cam-out of the driver from the recess when the fastener is driven. In many applications, it is very important that the fastener must be capable of withstanding several cycles, as in applications where the fasteners must be removed in order to repair or replace parts or to remove and replace access panels. The fastener drive system ideally should be capable of such repeated cycling, particularly in environments where the recess may become contaminated, painted, corroded or otherwise adversely affected in use. In such environments, it is essential that the drive system maintain driving engagement while applying torque in a removal direction. It may be necessary for the drive system to be capable of applying even higher levels of torque when removing the fastener, as may occur when the fastener is over-tightened during initial assembly, or where corrosion develops at the interface at the engaged threads, or if thermal cycling of the assembled components has placed increased stress on the fastener. These, and other, characteristics often present competing considerations; and compromises of one in favor of another may have to be made.
A variety of recess and driver configurations are in common use, including a number of cross-recesses, such as those described in U.S. Pat. No. Re. 24,878 (Smith et al.); U.S. Pat. No. 3,237,506 (Muenchinger) and U.S. Pat. No. 2,474,994 (Tomalis). Other fastener geometries include multi-lobe geometries of the type described in U.S. Pat. No. 3,763,725 (Reiland) and ribbed drive systems as described in U.S. Pat. No. 4,187,892 (Simmons). Also among the common recess configurations is the “Allen” system which is essentially a straight walled hexagonally shaped socket receptive to a similarly shaped driver. A fastener system having multiple lobes with spirally configured drive surfaces is described in U.S. Pat. No. 5,957,645 (Stacy).
With the exception of the ribbed systems, the walls and faces of the driver and recess typically are designed to fit closely with each other in an effort to achieve face-to-face contact of the driving and driven surfaces. With cross-recess fasteners, such face-to-face engagement can occur only, if at all, when the driver is properly aligned and seated within the recess. As a practical matter, however, in order to enable the driver to be inserted into the recess, there necessarily must be some clearance between the two.
The necessity for such clearance is even more critical with recesses having substantially axially aligned (straight) drive walls, as in the Reiland 725 patent and Allen head systems. In all of these systems, the practical result of the necessity for such clearance is that substantial face-to-face, broad area contact between the driver and recess surfaces is seldom achieved, if at all. With most drive systems for threaded fasteners, the driver mates with the recess in the head in a manner that results in point or line contact rather than face-to-face broad area contact. The actual area of contact typically is substantially less than full face-to-face contact. Consequently, when torque is applied by the driver, the forces applied to the screw head tend to be concentrated in localized areas with resulting high localized stresses and unstable axial alignment. Such localized high stress can plastically deform the recess, forming ramps or other deformations that result in premature, unintended disengagement of the driver from the recess.
A fastener system for maximizing the engageable surface area between the driver and drive surfaces is described in the Stacy'645 patent, which is commonly owned with the subject application. The disclosure of the '645 patent is incorporated in this application by reference. The recess and driver of the '645 patent are constructed with spirally configured engaging surfaces that are substantially aligned parallel with the axis of the fastener and may be classified generically as a straight walled fastener system. A more robust embodiment of the spiral drive fastener system is described in U.S. patent application publication 2009-0104002 (Dilling), commonly owned with the subject application. The disclosure of the Dilling application is also incorporated herein by reference.
The advantages of the invention described in the '645 patent are achieved by configuring the driving and driven surfaces of the driver and fastener, respectively, to conform to a segment of a spiral and, particularly, in a spiral configuration that enables a substantial and generous clearance between the driver and the recess during insertion and removal of the driver, but in which the fully seated driver is permitted to rotate to take up that clearance. The spiral configurations of the drive walls of the driver and the driver-engageable walls of the recess are such that when the spiral walls engage, they do so over a relatively broad area thereby applying and distributing the stress over that broad area. The spirally configured driving and driven walls are oriented to direct a major portion of the applied torque substantially normal to the fastener radius with little, if any, reliance on frictional, near-tangential engagement.
Another example of a straight walled fastener system is the system described in U.S. Pat. No. 3,584,667, issued to Reiland. This reference is incorporated herein by reference. The Reiland '667 patent describes a fastener system in which the driving surface geometries consist of a series of semi-cylindrical surfaces arranged substantially in the shape of a hexagon. The Reiland fastener systems are generically referred to as hex-lobular and have driving surfaces that are parallel with the axis of the fastener.
Although straight walled fasteners are in successful general use in many applications, they may experience difficulties resulting from axially misalignment between driver and fastener. In addition it has been difficult to obtain a reliable friction engagement that provides a stick fit feature. A stick fit feature is desired to hold the fastener on the driver in alignment, while the installation of the fastener is initiated. This is especially useful in high volume assembly line operations that use power driven bits to apply torque to the fastener. Axial alignment and stick fit are also important as the fastener length is extended.
In many applications in which a straight walled drive system is used, the driver may be power driven or required to be inserted in locations of limited access. In such situations, there is a need to releasably engage the fastener on the driver in advance of installation so that the driver can be used as an insertion tool, as well as a driver. This “stick fit” feature has been attempted in several different types of fasteners, for example, in fastener/driver systems having a cruciform (cross shaped geometry), several are shown in U.S. Pat. Nos. 6,199,455 and 4,457,654. A fastener system having a square drive geometry is illustrated in U.S. Pat. No. 4,084,478. It is observed that the stick fit efforts focus on the drive surfaces.
The “stick fit” feature allows the fastener to be releasably engaged on the driver to enable manipulation of the driver and fastener as a unit in hard to reach, automated, and other applications. Once installed, the fastener and driver may be disengaged with minimal effort.
The reference Larson, U.S. Pat. No. 4,269,246 is of interest in that it employs a partially tapered driver to enhance engagement. In Larson, the internal radius of the driver flutes are disposed parallel to the axis of the driver while the crest of the lobe is tapered inward toward the tip. The expressed purpose of this is to avoid premature interference between bit and recess. It is observed that the configuration results in a line contact between driver and recess both circumferentially and axially and will not enhance stability or frictional engagement. Only the bit is tapered with no change to the recess geometry.
Also of interest is the reference Goss, U.S. Pat. No. 5,461,952. In Goss a trailing side wall of the driver is tapered to provide a gradually thickening lobe geometry that generates a friction engagement on a drive surface. Since only one side wall is tapered the engagement with the straight sided drive surface becomes a circumferential line contact. Again only the bit is reconfigured. This is because there is a reluctance to alter the recess geometry as it would result in a loss of compatibility with existing drivers. Backward compatibility is a design advantage in any of the fastener systems, in particular straight walled systems.
A fastener system configured to provide stick fit in a straight walled fastener is described in the reference Dilling, U.S. Pat. No. 7,293,949, commonly owned with this application. In Dilling interference surfaces are constructed on inner non-driving transition surfaces between the wings of the fastener recess. It has been found that an improved stick fit feature may be obtained using a standard driver for this type of fastener system, using the interference surface on the so called “B” dimension of the recess.